/*
 * Copyright 2002-2005 The Apache Software Foundation.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * 
 * NOTE: This class has been stripped down to the Levenshtein distance 
 * algorithm alone (Amin Ahmad).
 */
package org.ahmadsoft.postal;


/**
 * A modified version of the original StringUtils.java to only include
 * the Levenshtein distance algorithm.
 */
class StringUtils {

    // Misc
    //-----------------------------------------------------------------------
    /**
     * <p>Find the Levenshtein distance between two Strings.</p>
     *
     * <p>This is the number of changes needed to change one String into
     * another, where each change is a single character modification (deletion,
     * insertion or substitution).</p>
     *
     * <p>This implementation of the Levenshtein distance algorithm
     * is from <a href="http://www.merriampark.com/ld.htm">http://www.merriampark.com/ld.htm</a></p>
     *
     * <pre>
     * StringUtils.getLevenshteinDistance(null, *)             = IllegalArgumentException
     * StringUtils.getLevenshteinDistance(*, null)             = IllegalArgumentException
     * StringUtils.getLevenshteinDistance("","")               = 0
     * StringUtils.getLevenshteinDistance("","a")              = 1
     * StringUtils.getLevenshteinDistance("aaapppp", "")       = 7
     * StringUtils.getLevenshteinDistance("frog", "fog")       = 1
     * StringUtils.getLevenshteinDistance("fly", "ant")        = 3
     * StringUtils.getLevenshteinDistance("elephant", "hippo") = 7
     * StringUtils.getLevenshteinDistance("hippo", "elephant") = 7
     * StringUtils.getLevenshteinDistance("hippo", "zzzzzzzz") = 8
     * StringUtils.getLevenshteinDistance("hello", "hallo")    = 1
     * </pre>
     *
     * @param s  the first String, must not be null
     * @param t  the second String, must not be null
     * @return result distance
     * @throws IllegalArgumentException if either String input <code>null</code>
     */
    public static int getLevenshteinDistance(String s, String t) {
        if (s == null || t == null) {
            throw new IllegalArgumentException("Strings must not be null");
        }
        int d[][]; // matrix
        int n; // length of s
        int m; // length of t
        int i; // iterates through s
        int j; // iterates through t
        char s_i; // ith character of s
        char t_j; // jth character of t
        int cost; // cost

        // Step 1
        n = s.length();
        m = t.length();
        if (n == 0) {
            return m;
        }
        if (m == 0) {
            return n;
        }
        d = new int[n + 1][m + 1];

        // Step 2
        for (i = 0; i <= n; i++) {
            d[i][0] = i;
        }

        for (j = 0; j <= m; j++) {
            d[0][j] = j;
        }

        // Step 3
        for (i = 1; i <= n; i++) {
            s_i = s.charAt(i - 1);

            // Step 4
            for (j = 1; j <= m; j++) {
                t_j = t.charAt(j - 1);

                // Step 5
                if (s_i == t_j) {
                    cost = 0;
                } else {
                    cost = 1;
                }

                // Step 6
                d[i][j] = min(d[i - 1][j] + 1, d[i][j - 1] + 1, d[i - 1][j - 1] + cost);
            }
        }

        // Step 7
        return d[n][m];
    }

    /**
     * <p>Gets the minimum of three <code>int</code> values.</p>
     *
     * @param a  value 1
     * @param b  value 2
     * @param c  value 3
     * @return  the smallest of the values
     */
    private static int min(int a, int b, int c) {
        // Method copied from NumberUtils to avoid dependency on subpackage
        if (b < a) {
            a = b;
        }
        if (c < a) {
            a = c;
        }
        return a;
    }

}